Systems for remotely controlling electrical devices from a manual control panel are well known. In some systems employing point-to-point input/output, or I/O, architecture, each of a plurality of manually activated operator interface devices, such as a push button, a key operator, a joy stick, or a rotary selector can be used to operate an associated contact block having one or more electrical contacts. Each contact block is electrically connected to an associated remotely located output device, such as a pilot light, a contactor, an overload relay or other electrically controllable device. These individual point-to-point connections between each contact block and its associated output device causes the task of making a manual control panel assembly extremely labor intensive and costly.
In order to reduce the labor costs as well as the cost of materials, it is now known to construct manual control panel assemblies wherein a number of operator interface controlled contact blocks are connected to a number of remotely located output devices by a single time division multiplexing communications link shared by all of the input and output modules of the system. In this system each contact block is electrically connected to an associated input access module which provides access to the multiplexing communications link over which the status of that particular contact block is conveyed to an associated output access control module. The output access control module is electrically connected to an associated output device which is ultimately controlled by the associated operator interface device at the control panel. This mode of operation is herein defined as the STAND-ALONE-MODE or PEER-TO-PEER mode where one contact block and its associated input access module sends a signal directly to its associated output access module for controlling the associated output device. This system could also be used in a HOST mode where the signal from the input access module is sent to a computer or other device having a CPU or means for adding simple logic functions such as AND, NAND, OR or NOR to the signal. A control system employing a time division multiplex common communication link is shown and described in U.S. Pat. No. 4,808,994 issued on Feb. 28, 1989 to Riley for "Logic Interchange System" and in the improvement patent application Ser. No. 08/099,788 filed Jul. 30, 1993 assigned to the assignee of the present invention and incorporated herein by reference. In this type of system, both the input and output access modules contain a communications circuit for communicating on the time division multiplexed common communications link and a circuit for implementing basic logic functions such as AND, NAND, OR and NOR. Because the logic circuit is included in each of the input and output access modules the system can operate in either the STAND-ALONE-MODE or the HOST mode. The state of the contact block is received by the input access module where it is processed and passed onto the communications link in the proper time slot to be received by the associated output access module. The output access module interprets the signal and passes the appropriate signal on to the output device for action.
It is also known to convey contact block status information and input access module mode status between a number of input access modules and their associated output access modules via network signaling. An example of such a network signaling control system is the LonWorks system which is assembled from components manufactured by Echelon Company of Palo Alto, Calif. The LonWorks system also comprises an ASIC which produces the signals for the network and can also be programmed with software to produce the basic logic functions of AND, NAND, OR and NOR.
The use of either of these two types of distributed I/O architecture will eliminate the point-to-point wiring between each contact block and its associated output device. However, in the current state of the art, a point-to-point connection is still required between the contact block and some type of input access module associated with the contact block and between the output device and some type of output access module associated with the output device. Each connection between an access module and a contact block or output device requires at least two separate wires and sometimes more. In control panel construction the operator interface device is installed on the outside of the panel cover and the contact block is on the inside of the panel cover. The operator interface device and contact block are in communication with each other such that manually operating the operator interface device will change the state of the electrical contacts within the contact block. The input access module is generally placed at an easily accessible location inside the control panel such as the back wall of the panel enclosure. This is usually some distance away from the contact block. It is also common for a control panel to include a number of pilot lights, each indicating the status of an associated output device controlled by its associated operator interface device. These pilot lights are actually output devices and therefore require a connection to the common communications link. Each pilot light requires a separate output access module and wiring between the output access module and the pilot light. The wiring between the contact blocks, pilot lights and their associated input and output access module must be of sufficient length to permit cover removal when inspection, maintenance or trouble shooting is required. This generally requires several feet of wire for each connection. Control panels which have a large number of operator interface devices and pilot lights are difficult to wire and assemble due to the many wires required. Thus, even with the common communication link between input access modules and output access modules the control panel itself still requires considerable wiring time and material. The physical size of the control panel enclosure is also a consideration. For each contact block and pilot light in the control panel, one input or output access module is required. The input and output access modules are at least the same size as the contact blocks and pilot lights and in many cases much larger. Therefore the volume of the control panel must be of sufficient size to enclose all of the input and output access modules and the wire required for connecting them to their respective contact blocks and pilot lights.